1. Field of the Invention
This invention is related to the general field of direct access computer disk drives used for storing computer data on rigid magnetic disks. In particular, the invention pertains to a gimbal used in a head assembly of a disk drive.
2. Description of the Related Art
The magnetic recording head of a rigid disk drive operates by flying in very close proximity over the surface of a magnetic disk, thereby accurately reading and writing data thereon. While the magnetic recording head, referred in the art as the "slider," is flying disposed substantially in parallel over the disk during operation, it must be able to adjust its attitude to conform to magnetic-disk surface imperfections and dynamic displacements, such as surface vibrations generated by the rotating movement. Therefore, the torsional characteristics of the flexure assembly supporting the slider are critical to the proper functioning of the apparatus and must be maintained within prescribed design specifications to prevent contact with the disk surface and avoid the disabling consequences that normally result therefrom.
As well understood in the art, the flexure assembly used in a conventional disk drive head assembly generally includes a gimbal, a load beam and a mounting plate. The gimbal is affixed to the slider, which contains an electromagnetic transducer for recording and retrieving information from the spinning magnetic disk. The slider is provided with a self-acting hydrodynamic air bearing that allows the slider to fly in close proximity to the disk, thereby improving the performance of the electromagnetic transducer. The gimbal is designed to provide compliance in the directions of the slider's pitch and roll axes, thereby allowing the slider to freely follow the topography of the magnetic disk. On the other hand, the gimbal is also designed to be non-compliant, or rigid, in the yaw, off-track (generally the direction transverse to the gimbal), and on-track directions, thereby ensuring that the electromagnetic transducer is maintained in precise alignment with the data track of the magnetic disk. In order to reduce cost, the gimbal is normally fabricated as a single discrete piece of metal foil, usually 304-series stainless steel, full-hard temper.
The load beam is affixed to the gimbal and the mounting plate and provides a precise spring force that counteracts the lifting force generated by the slider's air bearing, thereby ensuring that the flying height of the slider above the disk is maintained at the desired design target. The load beam must be compliant in the vertical or out-of-plane direction, so as to allow the slider to follow freely the topography of the disk. At the same time, the load beam is intended to be rigid in the horizontal or in-plane direction, thereby ensuring that the electromagnetic transducer is maintained in precise alignment with the data track of the magnetic disk.
The mounting plate, which is affixed to the load beam, is a robust and rigid structure through which the flexure assembly can be secured to a test fixture, to tooling equipment, or to a head positioning device in a consistent and predictive manner that will not impart damage to the relatively more delicate load beam. Of these three components, the gimbal presents the most significant design challenges to ensure the intended performance of a computer head assembly.
A conventional discrete gimbal 10 for a disk drive head assembly, such as described in U.S. Pat. No. 4,167,765 (Watrous), is illustrated in FIGS. 1 and 2. This head assembly encompasses several identifiable features. One is a slider supporting pad 12, which is coated with adhesive during the head assembly manufacturing process and secures the slider 14 to the gimbal. The supporting pad tends to be in the shape of a cantilever tab; thus, it is often referred to as "the tongue." Another feature is a load-point protrusion 16, which is semi-spherical in shape and located somewhere on the slider supporting pad. The load-point protrusion 16 allows for the load force from the load beam to be transmitted to the slider in a manner that does not inhibit movement of the slider in the pitch and roll directions. The load-point protrusion is often referred to as "the dimple."
The patented gimbal 10 includes a gimbal beam 18 disposed on each side of the slider supporting pad 12. The gimbal beams 18 are designed to be flexible in the out-of-plane direction to impart the desired pitch and roll compliance to the gimbal, while being rigid in the in-plane directions to impart the desired yaw and in-plane rigidity. An additional, lateral beam 20 connects the distal ends of the two gimbal beams 18 to the slider supporting pad 12. The lateral beam 20 is normally stamped out-of plane to offset the slider supporting pad away from the gimbal beams by an amount approximately equal to the height of the dimple 16, and to locate the slider supporting pad 12 in a particular angular position that will impart desired fly-height characteristics to the slider 14. The lateral beam 20 is often referred to as the "step form area."
A weld plate 22 connects the proximate ends of the two gimbal beams 18 and is laser welded to the load beam. The weld plate region normally has a circular tooling hole 24 adapted to engage a pin in a fixture used to align the gimbal to the load beam during laser welding. Typically, this tooling hole is about 0.800 mm (0.315 inches) in diameter.
The prior art comprises several other gimbal designs developed to fit particular applications and equipment, mostly based on variations of the Watrous patent. In spite of these improvements, the features of the gimbals used in modern flexure assemblies are problematic in a number of ways. The first problem arises from the requirement that the gimbal be compliant in the pitch and roll directions, and yet rigid in the yaw and in-plane directions. If the gimbal beams 18 are made longer, the pitch and roll compliance is improved but the yaw and in-plane stiffness is degraded. Likewise, if the gimbal is made from a thinner foil, or the width of each gimbal beam is decreased, the pitch and roll compliance is improved but the yaw and in-plane stiffness is reduced. Similarly, if the spacing between the gimbal beams is reduced, the roll compliance is improved but the yaw and in-plane stiffness is degraded. Thus, the design of gimbal beams based on the features of the Watrous patent necessarily represents a compromise between these competing characteristics.
Another problem with the patented gimbal is that its out-of-plane compliance can lead to slider air bearing damage during manufacture of the disk drive as a result of careless handling of the head positioning actuator device prior to installation into the disk drive. When the head positioning actuator device is being prepared for installation, the heads are kept out of contact with each other by using a comb-like tooling fixture with a plurality of fingers that deflect the load beams and separate the sliders. However, shock and vibrations produced by careless handling can result in a slider contacting an adjacent slider and damaging either or both sliders' air bearing surfaces, and possibly also the gimbals. Thus, the out-of-plane stiffness of the gimbal must be low for compliance in pitch and roll, yet it must be high enough to prevent excursions of the slider into another slider during moments of careless handling of the head positioning device.
Yet another problem with the gimbal 10 of the prior art is the step-forming construction of the lateral beam. Such forming unavoidably draws material out-of-plane, which results in the ends of the gimbal beams being drawn towards each other. Since the gimbal beams 18 are designed to have a high resistance to bending in-plane, the step-form operation leaves the gimbal in a state of high stress due to the tendency of the gimbal beams to return to their natural state. These high stresses tend to concentrate at the bend points 26 of the step form because of the relatively sharp radius of curvature at the bend points, and because of the reduction in material thickness of the gimbal at the bend points 26 caused by the forming step. Thus, cracking can take place in this region, often from fatigue when the gimbal is vibrated at very high frequency in an ultra-sonic cleaning system.
An additional common problem with conventional gimbals is the large size of the weld plate region 22. This introduces additional and undesirable mass to the flexure assembly. Furthermore, the large weld plate size is problematic when an existing gimbal design is to be used with a load beam that has been reduced in size for use with smaller sliders and disk drives. In such cases the gimbal will tend to be too large to fit onto the load beam, which then leads to undesirable compromises in the design of the load beam.
One more problem with conventional gimbals is the inclusion of the tooling alignment hole 24 in the weld plate region 22. The load beam has a corresponding tooling alignment hole. These holes are created by chemical machining (acid etching) of a foil sheet using well known industry processes and are held to very strict tolerances. During welding of the gimbal to the load beam, it is not uncommon for the etched edges of such holes to be in contact with the pins of the weld alignment fixture, which leads to pin wear caused by a scraping action when the flexures are removed from the weld fixture. When significant pin wear occurs, the through-hole diameter in the welded assembly may be significantly less than the original diameter of the gimbal tooling hole 24, or of the load beam tooling alignment hole. This condition can lead to an interference fit of the flexure assembly on subsequent tooling and fixturing pins, rendering the flexure assembly unfit for manufacturing even though the misalignment of the gimbal to the load beam may not be sufficient to render the flexure unfit for use, thereby creating unnecessary waste.
Still another problem with conventional gimbals is the way the slider supporting pad 12 causes distortion of the slider's air bearing surface. Because the stainless steel of the gimbal has a thermal coefficient of linear expansion different from that of the slider, changes in temperature produce unequal amounts of expansion (or contraction) in the gimbal and the slider, which leads to strain and distortion in both. The adhesive layer placed between the slider bonding pad and the slider is normally very thin in order to ensure coplanarity between the gimbal tongue and the backside of the slider; thus, a significant amount of thermally-induced stress and strain is transmitted between the slider and the gimbal. The resulting distortion of the air bearing surface can cause up to as much as a one-for-one change in the flying height of the slider, which results in the transducer being located too far from the disk and degrading the transducer's output, or in the slider flying too close to the disk thereby increasing the likelihood of catastrophic head-disk interaction.
One approach to reducing thermally-induced stress and strain is to decrease the area of the slider supporting pad 12. However, to ensure the manufacture of a well-formed load point protrusion 16, a sufficient area for clamping around the load point protrusion is required; thus, reducing this area can result in poorer quality of forming of the load point protrusion. Furthermore, it is desirable for the plane of the slider supporting pad to be flat and well defined to improve the predictability of the slider pitch and roll static attitude with respect to a given datum (e.g., the "flat" region in the rigid section of the load beam, or the mounting surface of the head assembly). Reducing the area of the slider supporting pad leads to flatness control problems because it is more difficult to clamp and hold the pad flat during load point protrusion forming.
Another problem with the slider supporting pad 12 disclosed in U.S. Pat. No. 4,167,765 is that its peripheral edges are located an exceedingly short distance from the inside edges of the two gimbal beams 18. If too much adhesive is applied by a head assembly production operator, the adhesive may squeeze out from under the slider supporting pad and bridge to one or both gimbal beams. This causes the gimbal to seize; that is, the gimbal can no longer freely move in pitch and roll, and the head assembly must be scrapped.
Still another problem with the slider supporting pad 12 is that the head assembly must be inspected during manufacture for insufficient adhesive (as well as for the excessive adhesive condition previously described). Inspection for adhesive fillets around the perimeter of the slider supporting pad is made difficult by the close proximity of the gimbal beams 18, the slider 14, and the load beam. This restricts the lines of sight to the slider supporting pad edges, and also reduces the amount of light that reaches this area to aid in the inspection.
In addition, the process of bonding a slider to a supporting pad of the type described in the prior art requires squeezing the adhesive between them to form very thin bondlines. The resulting thin and spread-out layer of adhesive restricts the utility of ultraviolet (UV) light curable adhesives, which are desirable for their quick cure times, because UV light is incapable of penetrating thin and broad adhesive films.
One more problem with the slider supporting pad 12 of the Watrous gimbal is that the adhesive-bond length in the transverse direction of the gimbal is substantially less than the width of the slider or the gimbal. The length of adhesive in this direction has a direct bearing on the strength of the bond between the slider and the gimbal.
Finally, another problem is the fact that the entire peripheral edge of conventional gimbals is extremely sharp due to the use of chemical machining to create the part and the nature of stainless steel. Since the electrical signals from the transducer are transmitted through soft and delicate copper wires that unavoidably must be routed across the edges of the gimbal, their sharpness can produce damaged wires that cause rejections during the manufacture of head assemblies.
Therefore, there is still a need for an improved gimbal design that addresses the problems associated with prior-art devices. The present invention is directed to a novel gimbal configuration that materially reduces these problems and enhances overall performance.